The present invention relates to a method and a device for checking patterns disposed on a material strip by photoelectric or other means, comprising control means and arrangements defined in the preambles of claims 1 and 9, and a material strip.
It is already known in the art to control processing operations on a travelling material strip by means of control marks printed on the strip which can be detected by photocells or other optical devices. These patterns which are disposed on the material strip are usually printed in a color tone which contrasts with the surroundings, thus enabling the detection means to detect the exact position of the patterns and therefore defining the current position of the material strip. These patterns can also be made with magnetically sensible material or with the help of patterns sensible by mechanical means, for example holes, crease lines, slots etc.
By the detection of the exact position of the material strip it is possible to apply multicolour printing on the material strip and to adapt the position of the pictures. These pictures contain several colours which are printed on the material strip by the usage of several processing machines, each of them printing a certain colour on top of the picture in process which has been prepared by preceeding processing machines which have printed other colours on the material strip. Therefore, a multicolour picture can be created and printed on the material strip wherein the different colours overlay each other at the exact position.
Another use of the checking device or checking method, respectively, is to detect the exact position of the material strip in order to apply a certain printing pattern on top of the previously preprinted material strip at a certain point or in order to arrange a printed pattern and a crease line pattern which facilitates the folding in register with each other.
Further fields of application exist, for example, where a packaging material strip is to be advanced in a packaging machine or the like over exactly the length of pattern so as to obtain, an the one hand, the printed pattern in the same position on all packages, and to ensure, on the other hand, that the said crease line pattern coincides with the forming device of the packaging machine so that the folding of the material takes place along the crease lines which are predetermined in their position. In order to manufacture the material strip, like stamping holes, applying opening apertures or additional crease lines, it is necessary to know the exact position of the material strip as well.
Up to know, it is well now to provide such material strips with patterns which can be detected by sensors, thus determining the exact position of the strip. It is known from EP-A-0 131 241 to use a pattern like a bar code with two groups of dark and light areas which succeed in the direction of the detection wherein the position of the dark areas and/or the light areas of one group with respect to a related detector is displaced in regard to the dark areas and/or the light areas of another group with respect to another related detector. These detectors detect the existence or, respectively, the non-existence of dark areas and/or light areas and produce output signals which depend on the result of the detection. This is carried out by using one detector which activates upon the detection of dark and/or light areas the other detector which detects the existence or the non-existence of dark and/or light areas wherein the output signals of the second detector are compared with a predetermined sequence of signals. If the sequence of output signals is equal to the predetermined sequence of signals the comparator will output an activation signal which activates the above described processing machines which require the exact position of the material strip.
These patterns usually disturb the general appearance of a decor of the material strip and it is desirable therefore, to accommodate the control patterns within the existing decor or on a very small blank surfaces which, in principle, only comprise the control patterns. However, this has given rise to great problems since the elements adapted to sense the patterns conceive and register all parts of the decor as well as the control patterns with the consequence that many output signals are supplied by the second detector depending on the dark and/or light areas of the decor and the control pattern. Since these output signals have to be compared within a real-time procedure it is quiet likely to detect false signals by an undesired identity of a sequence of dark and/or light areas of the decor and the predetermined sequence of signals. Consequently, the wrong position of the material strip is detected and the following processing machines destroy the decor or the material strip by getting the wrong information.
It has been another great problem that the control pattern should be as small as possible, in order to avoid the optical disturbance of the decor, but should avoid the detection of false signals.
Additionally, there is a need of the packaging industry to be independent from any control patterns and to detect the decor of the material strip by, for example, the decor itself.